Maria-Catalina Matei-Latiu; Calin Latiu; Tudor Papuc; George-Catalin Muntean; Adela Maria Daescu; Vasile Rus & Adrian Florin Gal

Fish gills represent one of the most vulnerable target organs to aquatic pollutants. Extensive research shows that chronic exposure to heavy metals leads to structural lesions and impaired ion regulation. However, up to the moment, it is unclear how such disturbances affect the countercurrent exchange mechanism, which is essential in fish respiration. The present study aimed to provide a thorough description of the histological and functional responses of gills to heavy metal exposure, explaining how such modifications affect the countercurrent exchange. Paired gill samples were harvested from adult European chub Squalius cephalus (Linnaeus, 1758) collected from the Arie? River catchment (geographical area of Rosia Montana), a high-risk aquatic ecosystem where fauna is continuously exposed to toxic concentrations of metals. 10 % buffered formalin fixed gill samples were further demineralized, processed with the usual paraffin embedding technique and stained with the Goldner’s trichrome method. The results showed a plethora of lesions, which were classified according to their magnitude into four different classes: adaptive, degenerative, inflammatory, and irreversible. The first three categories were present in almost all individuals, while the prevalence of the irreversible lesions was slightly lower. In such conditions, the countercurrent exchange mechanism is impaired even by initial modifications such as the lifting of the simple squamous covering epithelium. Due to a bigger distance between the lamellar axis and the exterior of the lamella, the oxygenated blood may find it harder to reach the destination point. Accordingly, the loss of lamellar structural integrity reduces the gradient necessary for optimal gas exchange, both protective adaptations and detrimental outcomes collectively undermining the physiological countercurrent exchange mechanism. These findings demonstrate that metal-induced histopathological damage might undermine countercurrent gas exchange, providing a reliable biomarker framework for assessing aquatic heavy metal toxicity.

KEY WORDS: Aquatic ecotoxicology; Gill morphology; Physiological mechanism; Respiratory efficiency; Sentinel species.

MATEI-LATIU, M. C.; LATIU, C.; PAPUC, T.; MUNTEAN, G. C.; DAESCU, A. M.; RUS, V. & GAL, A. F. Histopathological changes of the countercurrent system in fish gills constantly exposed to heavy metal-polluted waters. Int. J. Morphol., 44(3):1000-1007, 2026.